Dew Point Calculation for Multi-Component Mixtures

What is the Dew Point?

The Dew Point of a vapor mixture is the exact temperature (at a specific given pressure) where the vapor begins to condense. It is the thermodynamic state where the very first microscopic droplet of liquid forms.

As discussed in the bubble point article, multi-component mixtures (like natural gas) do not condense at a single temperature. They condense over a broad temperature range. The Dew Point is the upper boundary of this temperature range.

The Thermodynamic Criterion for the Dew Point

At the dew point, the system is almost entirely vapor. The liquid fraction ($L/F$) is mathematically assumed to be zero, but there is just enough liquid present to establish Vapor-Liquid Equilibrium (VLE).

Because it is mostly vapor, the composition of the vapor phase ($y_i$) is exactly equal to the overall composition of the mixture ($z_i$). Therefore, $y_i = z_i$.

The mathematical definition of the dew point is that the sum of all the mole fractions in the newly forming liquid droplet must exactly equal 1.0:

Using the definition of the K-value ($K_i = y_i / x_i$), we substitute $x_i = y_i / K_i$:

How to Calculate the Dew Point Temperature

Just like the bubble point calculation, the dew point requires a trial-and-error iterative approach because K-values depend heavily on temperature.

1. Guess a Temperature: Make an initial guess for the condensation point.
2. Find K-values: Look up or calculate the K-value for every component at the given pressure and guessed temperature.
3. Calculate Liquid Fractions: Divide $y_i / K_i$ for each component.
4. Check the Sum: Sum all the calculated liquid fractions.
   • If the sum exactly equals 1.0, your guessed temperature is the Dew Point!
   • If the sum < 1.0, your guessed temperature is too high (the mixture is still entirely superheated vapor). Guess a lower temperature and repeat.
   • If the sum > 1.0, your guessed temperature is too low (bulk condensation has already occurred). Guess a higher temperature and repeat.

Dew Point vs. Bubble Point on a Phase Envelope

If you plot the Bubble Point temperatures and Dew Point temperatures for a mixture across a range of pressures, you create a Phase Envelope (a P-T diagram).

• The curve connecting all the bubble points is the Bubble Point Line. If the fluid is at a pressure and temperature to the left of this line, it is a subcooled liquid.
• The curve connecting all the dew points is the Dew Point Line. If the fluid is to the right of this line, it is a superheated vapor.
• The area directly between the two lines is the Two-Phase Region, where the fluid exists as a boiling mixture of liquid and vapor.
• The exact point where the bubble point line and the dew point line meet at the top of the curve is the Critical Point of the mixture.

Try the Calculator

In industrial practice, Dew Points are critical for natural gas pipelines. If the pipeline temperature drops below the Hydrocarbon Dew Point, liquids will condense and destroy compressors! Use the Flash Drum / Separator Calculator on this page to dynamically calculate phase equilibrium for standard mixtures!